1. Field of the Invention
The invention relates to fluid-flow-control valves, and specifically to a pressure-flow-compensated four-way servovalve, having a single stage with a balanced control member, for use in hydraulic systems.
2. Description of Prior Art
Heretofore, four-way hydraulic valves having a single balanced moving member with multiple disks connected by an axial shaft, and having annular rings interspersed between the disks to form thin clearance passages for metering radial flows across the disks, had been naturally unstable; normally, the moving member sought an extreme--rather than central--axial position. Any nonsymmetrical flows within the valve--normally producing control flow between the control ports of the valve--created directly proportional, unbalanced hydrodynamic forces, oriented both axially and unidirectionally upon the moving member. In order to properly regulate flows through the valve, the moving member had to be restrained against displacements caused by these destabilizing forces. For this reason, the moving member was either actively positioned, or passively restrained, against destabilizing movements from a balanced position, over its entire range of axial displacement.
External linkages, relatively stiff springs, and powerful motors were used to position the moving member against the substantial unbalanced forces present in this type of valve. Counterchecking springs counterposed against the unbalanced hydrodynamic forces were often used to restrain the moving member from decentering and destabilizing displacements; the greater the fluid-power output capacity of the valve, the larger were the internal unbalanced forces, and the stiffer were the springs needed to restrain the moving member. However, in order to ensure accurate valve operation, the springs had to be carefully matched so that the moving member would be centered within its range of displacement under null operating conditions: zero control flow between, and zero load pressure drop across, the control ports. The more accurately the springs were matched and counterbalanced upon the moving member--when centered within the valve--the smaller was the applied null bias force necessary to restore the valve to the null condition. However, the very narrow range of displacement available in this type of valve severely reduced the acceptable error in centering the moving member at null. In order to achieve the necessary centering accuracy, the springs had to be precisely matched. Moreover, minimizing the null bias force became even more difficult as valve power output capacity increased: stiffer springs needed to counter the larger unbalanced forces were more difficult to match. Furthermore, since the springs opposed not only displacement of the moving member by unbalanced forces, but also its displacement by forces applied to actuate the valve, increasing their stiffness necessitated the use of more powerful force motors. And while the use of stiffer springs, together with more powerful motors, could augment the fluid-power output capacity of the valve, more powerful input signals were then needed to control the more powerful motors. Such motors were usually large and thus more easily located externally to the valve, being connected to its moving member by a mechanical linkage. However, the use of an external mechanical linkage, together with the external seals required to prevent fluid leakage from the valve, often proved awkward for use in servomechanisms. For these reasons, and since internal motors usually proved too weak to provide the applied force necessary to control the position of the moving member, this type of valve has not been widely used. If the unbalanced forces could be eliminated, obviating the need for restraining springs, smaller motors could be used to position the moving member without loss of fluid-power output capacity from the valve--thereby creating an improved balanced single-stage servovalve for use in electrohydraulic servomechanisms.